Wasmtime is a runtime for WebAssembly. From 25.0.0 to before 36.0.7, 42.0.2, and 43.0.1, Wasmtime with its Winch (baseline) non-default compiler backend may allow properly constructed guest Wasm to access host memory outside of its linear-memory sandbox. This vulnerability requires use of the Winch compiler (-Ccompiler=winch). By default, Wasmtime uses its Cranelift backend, not Winch. With Winch, the same incorrect assumption is present in theory on both aarch64 and x86-64. The aarch64 case has an observed-working proof of concept, while the x86-64 case is theoretical and may not be reachable in practice. This Winch compiler bug can allow the Wasm guest to access memory before or after the linear-memory region, independently of whether pre- or post-guard regions are configured. The accessible range in the initial bug proof-of-concept is up to 32KiB before the start of memory, or ~4GiB after the start of memory, independently of the size of pre- or post-guard regions or the use of explicit or guard-region-based bounds checking. However, the underlying bug assumes a 32-bit memory offset stored in a 64-bit register has its upper bits cleared when it may not, and so closely related variants of the initial proof-of-concept may be able to access truly arbitrary memory in-process. This could result in a host process segmentation fault (DoS), an arbitrary data leak from the host process, or with a write, potentially an arbitrary RCE. This vulnerability is fixed in 36.0.7, 42.0.2, and 43.0.1.
The bug lies in an incorrect assumption made by the Winch compiler regarding 32-bit memory offsets stored in 64-bit registers — the upper bits of the register may not be cleared, which the compiler incorrectly assumes. This allows a properly crafted Wasm guest to read or write memory before the start of the linear memory region (up to 32 KiB) or after its end (up to ~4 GiB), regardless of configured guard regions. The problem affects aarch64 architectures (confirmed working proof-of-concept) and x86-64 (theoretical case). The vulnerability is possible only when using the non-default Winch backend (flag -Ccompiler=winch); the default Cranelift backend is not affected.
An attacker controlling a WebAssembly module running with the Winch backend can cause host process crash (DoS via memory protection violation), steal arbitrary data from its address space, or — with write operations — potentially achieve remote code execution (RCE) in the context of the host process.
Update Wasmtime to version 36.0.7, 42.0.2, or 43.0.1. If immediate update is not possible, as a workaround discontinue using the Winch backend and use the default Cranelift backend. Details available in the vendor repository: https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-xx5w-cvp6-jv83
Bytecodealliance Wasmtime in versions from 25.0.0 to before 36.0.7, from 42.0.0 to before 42.0.2, and from 43.0.0 to before 43.0.1, only with active Winch backend (-Ccompiler=winch). The default configuration with Cranelift backend is not vulnerable.
CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:XBytecodealliance Wasmtime
APPBytecodealliance43.0.025.0.0 – 36.0.7 (excl.)37.0.0 – 42.0.2 (excl.)
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